In the manufacture of glass containers, various types of defects may occur. It has heretofore been proposed to employ optical scanning techniques for inspecting such containers for defects which affect optical transmission characteristics of the container sidewall. In U.S. Pat. Nos. 4,378,493, 4,378,494 and 4,378,495, all of which are assigned to the assignee of the present application, there is disclosed a method and apparatus in which glass containers are conveyed through a plurality of positions or stations where they are physically and optically inspected. At one optical inspection station, a glass container is held in vertical orientation and rotated about its vertical central axis. An illumination source directs diffused light energy through the container sidewall. A camera, which includes a plurality of light sensitive elements, i.e., pixels, oriented in a linear array parallel to the vertical axis of container rotation, is positioned to view light transmitted through a vertical strip of the container sidewall. The output of each pixel is sampled at increments of container rotation, and event signals are generated when the magnitude of adjacent pixel signals differ by more than a preselected threshold level. An appropriate reject signal is thus produced and the defective container is sorted from the conveyor line.
The method and apparatus disclosed in the aforementioned patents, commonly referred to as the Sidewall Inspection Device (SID), have been found to be very effective and efficient for general automated inspecting and sorting of glass containers. However, some problems have been encountered in using the SID for detecting certain specific types of defects. For example, to enhance detection of refractive defects which are transverse to the container axis, such as ribbon tear defects, it has been proposed in U.S. application Ser. No. 424,687 filed Sept. 27, 1982, now U.S. Pat. No. 4,487,322, to direct a filtered source of diffused light toward the sidewall of the container which provides a longitudinal illumination intensity gradient which varies in the direction of the vertical strip field of view of the camera, and thus substantially parallel with the axis of the container. The intensity of light is sensed and defect signals are generated as a function of differences between intensities at successive light sensitive elements within the camera. Defective containers are then sorted from the conveyor line. The technique so proposed has been employed successfully for reliable and efficient detection of transverse ribbon tear defects and sorting of containers having such defects.
Problems have also been encountered in using the SID for distinguishing among types and sizes of defects. For example, the SID employs a wide source of light energy which is wide enough so that most refracted defects do not refract light enough to be visible as a dark spot on the bright background of the wide source. However, because opaque defects absorb light energy, they are visible as dark spots on the bright background of the wide source. In other words, the SID detects opaque defects but is generally blind to refractive defects. The instant invention is directed to the problem of detecting the presence of an opaque defect, while at the same time being able to detect refractive defects by optical enhancement and further distinguish between small refractive defects, such as small blisters in the container sidewall, and large refractive defects, such as large blisters. It is important in the manufacture of glass containers to be able to identify major refractive defects such as large blisters as well as opaque defects, both of which may become sites for incipient crack propagation leading ultimately to fracture of the container. On the other hand, small refractive defects, such as small blisters in the container sidewall, are commercially acceptable and should not be rejected. These problems have been especially prevalent in the manufacture of narrow-neck containers such as beer bottles. Sorting and rejecting containers having commercially acceptable defects is inefficient and increases the manufacturing cost. Therefore, there is a need for an improved technique for reliably detecting commercially unacceptable defects while distinguishing such defects from otherwise commercially acceptable defects, and then for sorting and rejecting only those containers having commercially unacceptable defects.